A. Field of the Invention
The invention is related to oxygen-fired furnaces. More particularly, the invention is related to methods and systems for increasing the efficiency of an oxygen-fired furnace.
B. Description of the Related Art
One important goal sought by those skilled in the combustion art is to increase the efficiency of the combustion process, such as by reducing capital costs, energy costs, fuel and oxidant costs, and operating costs. One method of doing so is through thermal energy recovery in furnaces. When implemented, it can often reduce energy and/or fuel requirements needed to operate the furnace. However, many of the various thermal energy techniques have their drawbacks. One of these drawbacks is financial. Capital and operating costs needed to implement the heat recovery technology must not outweigh the economic benefit of the heat recovery. Another of these drawbacks is related to safety, i.e., the technology must not create undue safety risks. Another of these drawbacks is technologically related. The designs, materials and controls must be adequate to put theoretical concepts into practice.
In air-fired furnaces, one approach for improving the furnaces' energy efficiency is by using the excess energy from the flue gas to preheat the combustion air using recuperators or regenerators. Recuperators transfer some of the heat from the flue gas to the combustion air in a heat exchanger, while regenerators accumulate some of the heat from the flue gas in a ceramic or refractory material for later preheating of the combustion air. Oxy-fired combustion has been questioned by many because of its supposedly high capital and operating costs related to oxygen production. While the above techniques have been successfully employed in some air-fired combustion furnaces, they would be relatively more difficult to apply to oxy-enriched combustion furnaces because of the hazards of handling the extremely reactive hot oxygen, thereby increasing the doubt about the financial soundness of oxy-fired combustion.
Cogeneration of power and heat by production of electricity and/or steam is another technique available to recover the thermal energy for uses other than recycling it back into the furnace. The disadvantage of this approach is that the capital costs often tend to be relatively high, thereby outweighing any economic benefits realized by this heat recovery strategy.
Another approach is thermochemical energy recovery, also known as fuel reforming. In fuel reforming, the heat content of a fuel is increased by reacting it with steam or carbon dioxide, or a mixture of the two in a reactor (reformer), to generate a combustible mixture of H2 and CO that has a higher heat content than the initial fuel. Because this endothermic reforming reaction occurs at high temperatures (typically 900° C.), it beneficially utilizes the high temperature of the flue gases to provide the energy needed for the reforming reaction. However, this approach has its disadvantages. For many furnaces, the fuel consumption is not high enough to justify the high capital cost of installing a fuel reforming system. The complexity of the reformer and the safety constraints associated with handling hot H2 and CO are additional drawbacks. If this technology were applied to oxy-fired furnaces, the complexity of the reformer would be multiplied because the energy available from the flue gas is typically not sufficient for reforming all of the fuel, thereby requiring an additional energy source in addition to the thermal energy of the flue gas.
The heat recovery methods used in conventional air-fired furnaces are often not readily applicable to oxy-fired (oxygen-enriched) combustion furnaces, because of process and handling difficulties. For example, flue gases from oxy-fired combustion are extremely high in water content (up to about 60%), and are at very high temperatures (1200° C. or higher). They may also contain high levels of particulates, condensable batch vapors, or condensed batch vapors, thereby making the flue gases quite corrosive.
At the same time, the flue gas must also be treated at a pollution abatement system in order to comply with the applicable environmental regulations. Since these pollution abatement systems, such as electrostatic precipitator or baghouses, can only be operated at a temperature of about 400° C. or below, it is necessary to cool this gas down beforehand.
Cooling the hot flue gas before treatment at the abatement system may be done by diluting the hot flue gas with ambient air. However, air dilution increases the amount of flue gas to be treated, thereby increasing the capitol and operating costs of the abatement system used to treat the cooled flue gas, and consequently, the size and complexity of the pollution abatement equipment system increases. The capital and operating expenses also go up as well.
Cooling the hot flue gas before treatment at the abatement system may be done by spraying it with water. However, water spraying increases the dew point of the flue gas, thereby increasing the risk of corrosive gas condensation in the abatement systems or in the exhaust duct. If this technology were to be implemented in an oxy-fired furnace, this problem would likely be magnified because the water content of the flue gas can be as high as about 60% by volume.
Thus, those skilled in the art will recognize there is a need for an improved process and system for increasing the efficiency of oxygen-enriched combustion systems through heat recovery. They will also recognize there is a need for such an improved method and system that integrates a variety of heat recovery strategies. They will further recognize there is a need for such an improved method and system that will decrease oxygen and/or fuel requirements. They will further recognize that there is a need for such a method and system that will reduce a power requirement needed for oxygen production. They will further recognize that there is a need for such a method and system that will reduce the size of pollution abatement systems needed for treating pollution contained in flue gases. They will further recognize that there is a need for such a method and system that will allow safe recovery of thermal energy. They will still further recognize that there is a need for such a method and system that will reduce capital and operating costs. Finally, they will recognize that there is a need for an integrated method and system that satisfies all of these needs.